|a|=2, a{x₁;y₁}
|b|=3, b{x₂;y₂}
cos(a+b)=(a*b)/(|a|*|b|)
cos120°=(a*b)/(2*3)
-1/2=(a*b)*6. a*b=-3
a-2b{x₁-2x₂;y₁-2y₂}
|a-2b|=√((x₁-2x₁*x₂)²+(y₁-2y₂)²)
((x-2x₂)²+(y₁-2y₂)²)=x₁²-4x₁*x₂+4x₂²+y₁¹-4y₁*y₂+4y₂²=
=(x₁²+y₁²)-4(x₁*x₂+y₁*y₂)+(x₂²+y₂²)=|a|²-4*a*b+|b|²
|a-2b|=√(2²-4*(-3)+3²)=√(4+12+9)=√25
|a-2b|=5